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6 assessment of climate change in the southwest united statesthan winter and spring (medium-high confidence) (Figures 1.5 and 1.6). Summerheat waves will become longer and hotter (high confidence). Winter cold snapswill become less frequent but not necessarily less severe (medium-high confidence).[Chapter 6 and 7]• Average precipitation will decrease in the southern Southwest and perhaps increasein northern Southwest. Precipitation will decline in the southern portionof the Southwest region, and change little or increase in the northern portion(medium-low confidence) (Figure 1.6). [Chapter 6]• Precipitation extremes in winter will become more frequent and more intense(i.e., more precipitation per hour) (medium-high confidence). Precipitation extremesin summer have not been adequately studied. [Chapter 7]• Late-season snowpack will continue to decrease. Late winter-spring mountainsnowpack in the Southwest will continue to decline over the twenty-first century,mostly because temperature will increase (high confidence) (Figure 1.7).[Chapter 6]• Declines in river flow and soil moisture will continue. Substantial portions ofthe Southwest will experience reductions in runoff, streamflow, and soil moisturein the mid- to late-twenty-first century (medium-high confidence) (Figure1.7). [Chapter 6]• Flooding will become more frequent and intense in some seasons and some partsof the Southwest, and less frequent and intense in other seasons and locations.More frequent and intense flooding in winter is projected for the western slopesof the Sierra Nevada (medium-high confidence), whereas snowmelt-drivenspring and summer flooding could diminish in that mountain range (high confidence).[Chapter 7]• Droughts in parts of the Southwest will become hotter, more severe, and morefrequent (high confidence). Drought, as defined by Colorado River flow amount,is projected to become more frequent, more intense, and more prolonged, resultingin water deficits in excess of those during the last 110 years (high confidence).However, northern Sierra Nevada watersheds may become wetter withclimate change (low confidence). [Chapter 7]1.4 Recent and Future Effects of Climatic Change inthe SouthwestTerrestrial and freshwater ecosystemsNatural ecosystems are being affected by climate change in noticeable ways, which maylead to their inhabitants needing to adapt, change, or move:• The distributions of plant and animal species will be affected by climate change.Observed changes in climate are associated strongly with some observed changesin geographic distributions of species in the Southwest (high confidence).[Chapter 8]